Retractable assemblies are manufactured and sold by the group of firms, Endress+Hauser, in a large multiplicity of variants, an example being “Cleanfit H CPA475”.
Retractable assemblies are widely used in analytical measurements technology and process automation. They serve to withdraw probes from the process, and therewith from the medium, without process interruption and then to reintroduce them back into the process. The probes are held in an immersion tube and moved by means of a drive manually or automatically, for example pneumatically, axially between a process position (measuring) and a service position (maintenance, calibrating, rinsing, washing, probe replacement, etc.). These procedures occur periodically or as a function of other determinable or measured parameters.
Probes in the sense of this invention comprise probes with at least one receptacle for at least one sensor for measuring one or more physical or chemical, process variables.
The fields of use of retractable assemblies for measuring physical or chemical, process variables of a medium, e.g. of a fluid, especially a liquid, in process technology are many. The sensors are used for determining the process variables. The sensors can be, for example, pH-sensors, conductivity sensors, optical or electrochemical sensors for determining a concentration of a substance, e.g. O2, CO2, certain ion types, organic compounds, or the like, contained in the medium to be monitored.
If retractable assemblies are used for holding the sensor for determining at least one process variable, the sensor can in the service position be checked, calibrated, cleaned and/or replaced, wherein the sensor is, in such case, located in the housing interior of the retractable assembly, in the so-called service chamber. In order that the medium not be contaminated by the calibration-, rinse/wash- or cleaning liquid, in the service position the service chamber is so sealed relative to the containment, in which the medium is located, that no exchange of medium/liquid can take place. Usually, there is located for this purpose on the media end of the housing of the retractable assembly a seal, which in interaction with the end region of the immersion tube prevents an exchange of medium/liquid. This seal is located on the immersion tube or in the housing (service chamber). The seal is often embodied as an O-ring. The groove for the seal is, in such case, rectangularly shaped in cross section. In interaction with the circularly shaped O-ring, gaps result.
The gaps and edges form dead spaces, where then particles can easily deposit and scale and/or biofilms can form. These are undesirable, since then the performance of the retractable assembly suffers. In the worst case, germs and the like collect, multiply and so contaminate the medium and therewith render it unusable.